The use of radiation and/or chemotherapy in clinical cancer treatments is often unsuccessful for a number of reasons. For example, there is considerable evidence that oxygen deficient tumor cells, known as hypoxic cells, may be resistant to radiotherapy. See, Alper, T., et al., Nature 178:978-979 (1956); Dische, S., et al., Br. J. Radiol., 56:251-255 (1983); Churchill-Davidson, I., et al., Lancet 1:1091-1095 (1955); and Bush, R. S., et al., Br. J. Cancer 37 (Suppl. III):302-306 (1978), the disclosures of which are incorporated herein by reference. Hypoxic tumor cells may also be preferentially spared by some commonly used anti-cancer agents because of their location in poorly vascularized areas of tumors or their cell cycle state as reported by McNally, N. J., Int. J. Radiat. Oncol. Biol. Phys. 8:593-598 (1982), the disclosure of which is incorporated herein by reference.
Efforts to overcome hypoxia in clinical treatments of cancer have led to the development of hypoxic cell radiosensitizers which substitute for oxygen in radiochemical reactions. In particular, clinical trials with the hypoxic cell radiosensitizer misonidazole (MISO), an electron-affinic 2-nitroimidazole, have shown some benefit in certain situations, as reported by Overgaard, J., et al., Progress in Radio-Oncology III, Proceedings of the 3rd International Meeting on Progress in Radio-Oncology, March 1985, 137-147, J.C.R.O., Vienna, 1987, the disclosure of which is incorporated herein by reference. However, MISO has shown significant neurotoxicity such that the total dose of the drug administered to a patient is limited.
Radiation sensitizers of this type are hypoxia selective, meaning that such compounds are considerably more toxic to hypoxic cells relative to aerobic cells. This differential hypoxic cytotoxicity is due to bioreductive activation of the drug under anaerobic conditions. For this reason hypoxia selective cytotoxins are also referred to as bioreductive agents. Bioreductive agents are therefore potentially exploitable in cancer therapy.
At the present time, at least three other 2-nitroimidazole derivatives are undergoing clinical evaluation as hypoxic cell radiosensitizers. Etanidazole (SR-2508), a neutral agent which is more hydrophilic than MISO, is less neurotoxic in experimental animals and can be administered to humans at about a threefold higher dose than MISO as reported by Coleman, C. N., et al., Int. J. Radiat. Oncol. Biol. Phys. 10:1749-1753 (1984), the disclosure of which is incorporated herein by reference. Roberts, J. T., et al., Int. J. Radiat. Oncol. Biol. Phys. 10:1755-1758 (1984) and Saunders, M. I., et al., Int. J. Radiat. Oncol. Biol. Phys. 10:1759-1763 (1984), the disclosures of which are incorporated herein by reference, report that pimonidazole (Ro 03-8799) contains a basic piperidine moiety and has a total dose limitation similar to that of MISO. RSU-1069 is a bifunctional molecule containing a 2-nitroimidazole group and an alkylating aziridine. In experimental systems the RSU-1069 bioreduction agent has substantially greater activity than MISO as reported by Adams, G. E., et al., Br. J. Cancer 49:571-577 (1984); Stratford, I. J., et al., Int. J. Radiat. Biol. 55:411-422 (1989); Nias, A. H. W., Int. J. Radiat. Biol. 48:297-314 (1985); and Ahmed, I., et al., Int. J. Radiat. Oncol. Biol. Phys. 12:1079-1081 (1986), the disclosures of which are incorporated herein by reference. For example, Adams, G. E., et al., Br. J. Cancer 49:571-577 (1984) reports that bioreductive agent RSU-1069 is toxic to hypoxic cells in vitro at about a 100-fold lower concentration when compared to the toxic concentration for aerobic cells. The high differential cytotoxic activity of RSU-1069 is believed to be due to the formation of a very reactive, bifunctional electrophile upon the compound being reduced in cells.
Bioreductive radiosensitizers have also been shown to significantly enhance the activity of several chemotherapeutic agents, such as, for example, cyclophosphamide, nitrosoureas, and L-phenylalanine mustard in vitro and in vivo as reported by McNally, N. J., Int. J. Radiat. Oncol. Biol. Phys. 8:593-598 (1982); and Sieman, D. W., Int. J. Radiat. Oncol. Biol. Phys. 10:1585-1594 (1984), the disclosures of which are incorporated herein by reference. This enhancement of chemotherapeutic activity is known as chemosensitization or chemopotentiation.
Currently there is increasing interest in targeting bioreductive agents to DNA in order to improve the cytotoxicity of such agents. See Brown, J., Int. J. Radiat. Oncol. Biol. Phys. 16:987-993 (1984); and Nias, A. H. W., Int. J. Radiat. Biol. 48:297-314 (1985), the disclosures of which are incorporated herein by reference. As reported by Stratford, M. R. L., et al., Int. J. Radiat. Oncol. Biol. Phys. 16:1007-1010 (1989), the disclosure of which is incorporated herein by reference, it is the concentration of sensitizer within DNA as opposed to the average intracellular concentration which appears to be the major factor that determines cytotoxic efficacy. One such DNA targeting effort involves combining an alkylating agent with bioreductive functional groups within the same drug. See, for example, Adams, G. E., et al., Br. J. Cancer 49:571-577 (1984). Other efforts of groups such as Chibber, R., et al. Int. J. Radiat. Oncol. Biol. Phys. 10:1213-1215 (1984); Teicher, B. A., et al., Int. J. Radiat. Oncol. Biol. Phys. 11:937-941 ( 1985); Skov, K. et al., Int. J. Radiat. Biol. 52:289-297 (1987); Chan, P. K. L., et al. Int. J. Radiat. Oncol. Biol. Phys. 12:1059-1062 (1986); and Chan, P. K. L., et al., Int. J. Radiat. Biol. 52:49-55 (1987), the disclosures of which are incorporated herein by reference, employed transition metal coordination complexes such as platinum and ruthenium to target nitroaromatic radiosensitizers to DNA. However, as reported by Butler, J., et al., Radiation Research 102:1-13 (1985), the disclosure of which is incorporated herein by reference, these metal coordination complexes are often less effective as radiosensitizers than the free radiosensitizer molecule, despite the fact that the one electron reduction potential (a chemical property related to sensitization efficiency) can be increased in some platinum complexes compared to the free sensitizer ligand.
Another approach for targeting bioreductive agents to DNA involves use of an intercalating moiety such as phenanthridine or acridine which is inserted between the base pairs of the DNA. For example, NLP-1, 5-[3-(2-nitro-1-imidazoyl)-propyl]-phenanthridinium bromide, a 2-nitroimidazole-linked phenanthridine, has been synthesized and its hypoxic cell cytotoxicity and radiosensitization has been estimated by Panicucci, R., et al., Int. J. Radiat. Oncol. Biol. Phys. 16:1039-1043 (1989), the disclosure of which is incorporated herein by reference. Nitracrine, 1-nitro-acridine, is a potent hypoxia selective cytotoxin and a radiation sensitizer in mammalian cell culture, however, its rapid metabolism limits its radiosensitization efficacy in vivo. See, Wilson, W. R., et al., Br. J. Cancer 49:215-223 (1984); Roberts, P. B., et al., Int. J. Radiat. Biol. 51:641-654 (1987); Roberts, P. B., et al., Radiation Research 123:153-164 (1990); and Wilson, W. R., et al., Int. J. Radiat. Oncol. Biol. Phys. 12:1235-1238 (1983), the disclosures of which are incorporated herein by reference.
An acridine conjugated hypoxia selective cytotoxin that is relatively stable in vivo is desirable since acridine is a better intercalator than phenanthridine, as reported by Roberts, P. B., et al., Int. J. Radiat. Biol. 51:641-654 (1987). Non-covalent binding to DNA, such as by intercalation, may allow migration of the radiosensitizer to DNA sites where radiation induced radicals are created. Thus, as reported by Roberts, P. B., et al., Radiation Research 123:153-164 (1990) 1-nitracrine which exhibits faster dissociation kinetics of its DNA-drug complex than the other nitroacridine isomers is a 20-fold more potent sensitizer than other isomers tested.
Accordingly, there is a need for bioreductive activated cytotoxins which enhance the cytotoxic activities of ionizing radiation and chemotherapeutic agents to hypoxic cells without substantial aerobic toxicity.